Background[]
Not much is known about Katsuyama's early life, however, based on observations of his id well by Uraido, it is heavily implied that his childhood was a cruel, harrowing one, enough to mould him into a twisted, violent sadomasochist. At some point, he entered Kiki Asukai's dreams, where he met John Walker, who invited him to take out his violent urges on Asukai. Katsuyama accepted, killing her several times over in her dreams before turning his attention to real-life people, kidnapping them and holding them prisoner in an arena under his house where he would fight and kill them, going on to kill six people, including a martial artist and a pro-wrestler, all on Sundays, as per John Walker's manipulation.
Eventually, he would kidnap Asukai in real life to kill her. Taking her to his mansion, he began to beat her, however before he could actually kill her, he was interrupted by police officer Akihito Narihisago, the father of his previous victim, Muku Narihisago. Storming Katsuyama's house, Narihisago immediately shot and killed him, resulting in his own arrest.
ID: INVADED[]
FILE: 07 Thunderbolted[]
Katsuyama is mentioned by Kokuryu Matsuoka as he and two other field agents investigate his house two years after his death to obtain Akihito Narihisago's cognition particles. Miyo Hijiriido later investigated Katsuyama's id well, finding John Walker inside it, proving Kastuyama had contact with him.
FILE: 09 Inside-outed[]
A recreation of him appears in the "well within the well" that Narihisago finds himself in based on the past of the present world where his wife and daughter are still alive. Retaining full memories of the present, Narihisago visits Katsuyama in his home knowing he is "The Challenger" and formally challenges the latter inside his manor.
Kastuyama beating Narihisago
Katsuyama lets him pass admitting to his crimes but confused to the future events that Narihisago is alluding to. While inside the manor, he takes his clothes off leaving him in fighting gear as Narihisago attacks the serial killer, confronting him as the pervert who killed a 14-year-old girl. Katsuyama soon proves to be the superior fighter, knocking him down and breaking Narihisago's left arm, while still confused at the crimes Narihisago is accusing him of and mocking his lack of skill. Narihisago however uses his gun to shoot at his thighs, to which Katsuyama wondered if they weren't going to fight like adults. Narihisago retorts that he merely said they'd do it "Katsuyama's way" at one vs one.
Narihisago kicks Katsuyama in the face and tries to bash him with his weapon, but is blocked and nearly choked to death by Katsuyama through a leg lock which he escapes by being able to fire his weapon. The two briefly pause, relishing in the high of combat before Katsuyama locks himself to Narihisago's leg trying to break it with the latter trying to bash his head with his gun. Katsuyama blocks this and proceeds to brutally smash his fist on Narihisago's face repeatedly while laughing in joy. A barely conscious Narihisago manages to shoot him in the abdomen, further weakening Katsuyama.
Katsuyama's corpse
Katsuyama gets in position and tries to rush Narihisago, but receives a knee strike on his face instead, followed by a headbutt that knocks out his teeth. Narihisago then proceeds beat Katsuyama to death via repeated blows to the head, smashing it to pieces. Narihisago berates his dead opponent for not living up to his namesake and yells in victory after confirming his death.
FILE: 13 Channeled II[]
Katsuyama in his id well
As Uraido travels from id well to id well within his own well, he eventually ends up in Katsuyama's: a bath full of blood and teeming with sharks, with a giant fetus representing Katsuyama in the middle. Perching on his head, Uraido remarks that Katsuyama's first memory must have been a cruel one.
At that moment, Narihisago (in Sakaido's body) attacks Uraido, which disturbs Katsuyama, who begins screaming. As the two fight, Katsuyama tries to squash them, but they both dodge at the last minute, confusing him. He spends a moment trying to figure out where they went, before returning to a docile state once more.